Most airlines have a same day flight change option and, in my experience, a lot of people take advantage of it. This means that on the day of departure the flight loads are going to be very fluid until check in is cut off. My daughter-in-law got stuck in that just yesterday and spent the day at the airport trying to get home while I watched the flight loads shift and change on every flight.

Again, I'm not sure of all airlines' policies on the same day changes (some allow online changes, some require it to be at the airport) but the ones like Alaska who allow the passenger to change their own ticket to another flight that day generally allow the process to start at midnight. This would explain why you see a vast difference in loads during the middle of the night. Throw in the elite factor (they normally get a much bigger window to swap flights than the general public) and you have no way of knowing how something is going to look until you get really close to departure.